A generated code amount accumulation part adds up the amounts of generated codes of pictures in 1 GOP which are encoded up to the current stage. An upper limit code amount accumulation part adds up the upper limit amounts of codes of the pictures in the 1 GOP which are encoded up to the current stage. A transmission load of an image transmission system is taken into consideration in the setting of the upper limit amount of codes. An update ratio setting part outputs an update instruction to lower a target rate when the accumulated amount of generated codes exceeds the accumulated upper limit amount of codes. The update ratio setting part does not output the update instruction for lowering the target rate when the accumulated amount of generated codes does not exceed the accumulated upper limit amount of codes. A transcoder can predict whether or not there is a possibility that the load of transmitting image data will increase while each picture in 1 GOP is encoded.